Optical fiber has found widespread application in longhaul transmission such as, for example, between cities. It is readily apparent that optical fiber can be used not only in longhaul applications but also in local area networks, service or drop applications, and in indoor cable. As a result, end to end optical communication is fast becoming a reality.
Optical fiber cables designed for longhaul applications typically are not well suited for use in the local distribution network. For instance, such cables typically are designed to have a medium to relatively high fiber count, are frequently relatively rigid and have a relatively large bending radius, and tend to be relatively costly to manufacture. On the other hand, cable for use in local distribution networks should have a low fiber count, e.g., less than about ten fibers, should be flexible, should be usable in a variety of environments, should be installable by existing procedures and equipment and should be easy to manufacture and low in cost.
Notwithstanding the rapidly escalating use of optical fiber, there remains a need for metallic conductors such as copper conductors. Metallic conductors may be sought after to provide power or may be used for other low voltage signalling purposes. Furthermore, there may be a need for both optical fiber and metallic conductors at the same end use point. One such application may be at office workstations in a computerized facility.
Such a need translates into a need of a hybrid cable which is intended to refer to a cable which includes both optical fiber and metallic transmission media. The sought after cable should have relatively high tensile axial loading capability, have sufficient flexural stiffness to minimize bend losses, and inasmuch as it most likely will be used on customers' premises, it should be suitably flame retardant. Also, the sought after cable should be relatively easy to manufacture and to connectorize despite the presence of different kinds of transmission media.
Hybrid cables are known to the prior art. For example, described in F. J. Mullin - W. S. Reed U.S. Pat. No. 4,852,965 which issued on Aug. 1, 1989 is a hybrid cable which includes a reinforced optical fiber unit. The unit includes an optical fiber or fibers enclosed by at least three impregnated fiberglass members and a jacket. The unit is included along with metallic conductors in a core tube enclosed in a plastic jacket.
Also known are other hybrid cables in which optical fibers along with a twisted pair or pairs of metallic conductors are enclosed in a jacket. Such an arrangement may cause problems when the cable is routed in arcuate paths, such as might be expected in an office environment, because of the lack of control over the neutral axis of bending. Also, such an arrangement becomes somewhat difficult to terminate because of the randomness of the fiber and the metallic conductors in a core of the cable.
In hybrid cables, bending of the cable may aggravate buckling of cable components because of what seems to have been insufficient attention to the location of the neutral axis of bending with respect to vulnerable portions of a cable cross section which is transverse to a longitudinal axis of the cable. In bending, the neutral axis positions itself to cause the stresses across the cross section in tension to equal those in compression. The neutral axis will shift from the geometric center toward the more flexurally rigid side of the cross section to provide more area on the less flexural rigid side to balance the stresses. The result may mean that a significant portion of the cable cross section is stressed excessively. For an optical fiber cable or a hybrid cable which includes optical fibers this may mean that one or ones of the fibers undesirably may be excessively stressed. This also may occur in any composite cable, that is in any cable which is not homogeneous in materials throughout its cross section.
What is sought after and what does not appear to be available in the prior art is a composite cable which includes a hybrid cable, for example, for use in a workstation environment and one which may be routed in an arcuate path or paths without damaging the fiber or the metallic conductors. Further, the cable must be easily manufacturable and must be easily connectorized. The ability to automatically connectorize the cable would be a desired feature. Also, the sought after cable desirably is one in which the neutral axis is in a desired location with any optical fibers therein being subjected only to minimal stress.